1. Field of the Invention
The present invention relates to an ink jet printer in which drops of ink are jetted out onto a surface of a recording medium so as to record an image, and also relates to a linear actuator to be applied to this ink jet printer.
2. Description of the Related Art
An ink jet printer is characterized in that its weight is light and its structure is simple. In general, an ink jet head in which a large number of ink jet mechanisms are arranged close to each other in a small region is scanned in the lateral direction of a recording sheet, that is, in the primary scanning direction. Since a relatively heavy ink jet head is suddenly reversed on both sides in this system, a large amount of driving energy is required. The required energy is supplied by an electric motor, and it is not easy to accumulate the supplied energy so as to utilize it when the movement of the ink jet head is reversed. Since a jet of ink is stopped during the reversal of the ink jet head, time is wasted. It is preferable to complete the reversal of the ink jet head in as short a time as possible. Therefore, it is necessary to suddenly accelerate the ink jet head. Accordingly, the apparatus is given a reaction force by the sudden acceleration. Due the angular moment caused by the reaction force, vibration tends to be generated.
Conventionally, the following measures are taken to solve the above problems. For example, according to Japanese Examined Patent Publication No. 51-48743, an ink jet head supported by a spring is moved by an electromagnetic force so that scanning can be conducted on a surface of a recording sheet by the ink jet head. According to Japanese Examined Patent Publication No. 63-54552, in order to cancel an influence of the reaction force, a counter weight is moved in a direction opposite to the moving direction of the ink jet head. However, according to Japanese Examined Patent Publication No. 51-48743, when a single ink jet head is moved, a reaction force is given to a base portion of the support spring. Therefore, it is necessary to provide a base having a sufficiently high rigidity and mass. As a result, the apparatus becomes heavy and large. According to Japanese Examined Patent Publication No. 63-54552, the apparatus is composed in such a manner that a simple counterweight is guided by a slide shaft. Therefore, the mechanisms of a guide bearing and a drive cam become complicated, and energy is wasted to drive an additional mass.
According to Japanese Examined Patent Publication No. 2-31543, an optical sensor and an ink jet head arranged in line with each other are supported by a spring, and a plunger magnet is used as a drive source. According to Japanese Unexamined Patent Publication No. 5-138971, an ink jet head is supported by an elastic body, and back and forth movement is generated in the ink jet head by an urging mechanism directed to the center of amplitude.
In the cases of Japanese Examined Patent Publication No. 2-31543 and Japanese Unexamined Patent Publication No. 5-138971, a reaction force exerted by the moving body is received by a base. Therefore, the vibration generating force is not reduced, so that a base having a sufficiently high rigidity and mass is required. As a result, the apparatus becomes heavy.
In order to increase the printing speed of this type ink jet printer, it is necessary to use a printing head provided with a large number of nozzles in the width direction of the recording sheet and to move the printing head back and forth in a direction perpendicular to the recording sheet feed direction.
An ideal reciprocating motion for this type of printer, if slotted with time on the horizogal axis and distance on the vertical axis, forms a saw-tooth-wave. In other words, it is preferable to control an actuator in such a manner that the ink jet head advances at a constant speed and returns at an infinite speed or a very high speed.
However, in this case, a very high acceleration is required at the beginning of the motion. Especially, at the beginning of the returning motion, a very high acceleration is required. Accordingly, even when either a movable coil type or a stationary coil type linear actuator is used, a strong force is applied to the base, causing vibration in the base. The most natural reciprocal movement is a sine wave movement provided when a movable coil or stationary coil is given a sine wave current, or sine wave vibration provided by the combination of a spring and mass. However, even in this case, a high acceleration is generated at the turning point of the reciprocal motion, and a strong force is given to the base in the same manner as described above. When the strong force is given to the base like this, the following problems may be encountered. When the base is light or the rigidity is not sufficiently high, the base itself will vibrate, and in the worst case, the motion of the printing head will be hindered by the vibration of the base, and it is impossible to attain a sufficiently high printing accuracy.
An object of the present invention is to provide an economical ink jet printer in which a reaction force generated in the reciprocal motion of the ink jet head is simply canceled, and the ink jet printer is small and light, and less vibration is caused in the printer, and further it is possible to drive the printer with a small amount of drive energy.
The present invention has been proposed to solve the above problems of the conventional ink jet printers. Another object of the present invention is to provide an actuator for driving a printing head and other units which must be driven back and forth, characterized in that less vibration is given to the base, and the structure is simple and further there is no possibility of breaking wires.
In order to accomplish the above objects, as illustrated in FIG. 1 showing an embodiment of the present invention, an ink jet printer of the present invention comprises: a recording medium conveyance means for conveying a recording medium in a subsidiary scanning direction; a plurality of ink jet heads in which a plurality of nozzles for jetting drops of ink onto a surface of the recording medium are arranged, the plurality of ink jet heads being capable of moving in a primary scanning direction substantially perpendicular to the subsidiary scanning direction, the plurality of ink jet heads being arranged in the subsidiary scanning direction; and an ink jet head reciprocal means for reciprocating each ink jet head in the primary scanning direction at a different phase. It is preferable that at least a pair of ink jet heads be provided and driven by the ink jet head reciprocal means with phases directly opposite to each other.
When the plurality of ink jet heads arranged in line in the subsidiary scanning direction are reciprocated in the primary scanning direction at the different phases, it is possible to cancel the reaction force generated by the reciprocal motion of each ink jet head. In this case, when at least one pair of ink jet heads is provided and the pair of ink jet heads is driven with directly opposite phases, the reaction forces generated by the reciprocal motions of both ink jet heads can be completely canceled.
A linear actuator for driving the heads of the ink jet printer comprises: a stationary coil; and two permanent magnets, wherein opposite magnetic poles are opposed to each other on both sides of the stationary coil, so that a magnetic field is formed in a direction perpendicular to the axial direction of the coil, and at least one of the permanent magnets is movable. When a current is allowed to flow in the stationary coil, forces generated by the current and magnetic fields formed by the permanent magnets move the permanent magnets in the opposite direction to each other. Consequently, a force transmitted to the base from one of the permanent magnets is canceled by a force F transmitted to the base from the other permanent magnet. Therefore, the occurrence of vibration of the base can be prevented.